A variety of aerospace, civil and mechanical structures are typically instrumented with sensors, such as strain sensors, for measuring various forces or other physical phenomena to which the structure is subjected. For example, a number of steel and concrete structures, such as buildings, bridges, culverts, and tunnel linings, often include embedded strain sensors. In addition, while a variety of composite structures already incorporate strain sensors, the number of composite structures that will include sensors is expected to increase dramatically as composite structures are increasingly utilized in the aerospace, civil, construction, marine and transportation industries.
As known to those skilled in the art, the fabrication of composite structures generally subjects the composite structure to relatively high temperatures and relatively large pressures. An increasing number of composite structures are being pultruded so as to reduce the requisite manufacturing costs and time. As also known to those skilled in the art, a conventional pultrusion process wets the fibers with a resin prior to pulling the wet fibers through a heated die which subjects the wet fibers to relatively high temperatures and significant compressive forces. As such, sensors are particularly useful in conjunction with composite structures since the sensors will not only sense strain and other physical phenomena acting upon the composite structures following installation, but the sensors can also monitor strain and other physical phenomena imparted to the composite structures during the fabrication process. It is very advantageous to include sensors within pultruded composite structures so as to monitor the strain and other physical forces imparted to the composite structures during the pultrusion process and, in particular, during the curing of the resin within the heated die.
Traditionally, electronic sensors, such as electronic strain gauges, have been utilized to monitor the forces and other conditions, such as strain, to which an associated structure was subjected. In this regard, conventional electronic strain gauges include a resistive coating printed on a polymer substrate which is then attached to the structure, such that the resistance of the electronic strain gauge increases as the electronic strain gauge is stretched, thereby providing a measurement of the strain to which the associated structure is subjected.
In addition, piezoelectric sensors have also been embedded in structures, such as composite structures. As described by U.S. Pat. No. 5,305,507 to George R. Dvorsky, et al., a piezoelectric actuator or sensor can be encapsulated in a non-conductive fiber composite material formed of fiberglass cloth and a two-part epoxy. Once encapsulated, the actuator or sensor can be disposed within a composite structure. As described by the Dvorsky '507 patent, the actuator or sensor is encapsulated by covering the sensor or actuator with a fiberglass cloth and two-part epoxy and by placing the encapsulated sensor or actuator in a vacuum bag to extract unwanted air and excess resin. As such, the resulting shape of the encapsulated piezoelectric sensor or actuator is limited. That is, the encapsulated sensor would generally not be able to have an elongate rod-like shape, can only be formed into a planar or slightly curved shape in order to conform to the underlying structure and cannot have a number of other physical shapes that would be desirable for measurement purposes. In addition, the encapsulated piezoelectric sensor or actuator is generally relatively large in comparison to the subcomponent of the structure in which the piezoelectric sensor or actuator is embedded as well as in comparison to fiber optic and other types of sensors.
More recently, fiber optic sensors have been utilized to measure strain and other physical phenomena to which a structure is subjected. Fiber optic sensors are superior to comparable electronic sensors in a number of respects. As will be apparent, fiber optic sensors are much smaller than comparable electronic sensors. In addition, fiber optic sensors are less susceptible to electromagnetic interference, have improved corrosion resistance, reduced cabling requirements, have less physical influence on the overall structure, and generally improved measurement sensitivity.
However, fiber optic sensors also suffer from a number of shortcomings. For example, while the relatively small size of fiber optic sensors is advantageous in many respects, the small size makes fiber optic sensors relatively difficult to handle. In addition, fiber optic sensors provide an extremely localized measurement, such as a localized strain measurement. Unfortunately, engineers or other structural analysts oftentimes desire a measurement that has been averaged over a longer length or a larger area.
Additionally, fiber optic sensors are quite delicate. As such, the process for fabricating a carrier or other structure which includes a fiber optic sensor and/or the subsequent process of installing the carrier on or within a structure may damage the fiber optic sensor. Accordingly, at least some fiber optic sensors have been inserted into and bonded within a metal tube which is thereafter attached to or embedded within a structure, such as a concrete or composite structure. As such, the fiber optic sensor is somewhat protected by the metal tube from indelicate handling and forces present during the fabrication and installation processes that could otherwise be destructive.
In an attempt to enhance the mechanical bond between the sensor and the structure in which the sensor is embedded and to average the strain over the tube length, the metal tube generally has flared ends to create a dumbbell-like shape. Unfortunately, even dumbbell-shaped metal tubes are frequently relatively incompatible with the host material of the structure in which the sensor is embedded. For example, metal tubes oftentimes fail to form a secure bond with host material of the resulting structure. In addition, the coefficients of thermal expansion of the metal tube and the host material are also generally quite different such that the metal tube will expand and contract in different amounts than the host material as the temperature increases and decreases, respectively. Not only do the strains imposed upon a fiber optic sensor by the different coefficients of thermal expansion tend to adversely affect or alter the measurements provided by the sensor, but the differences in thermal expansion and contraction can destroy the bond, if any, between the host material and the metal tube and, consequently, between the host structure and the fiber optic sensor. Additionally, the metal tube is subject to corrosion when used in civil and concrete structures.
Thus, although a variety of structures, including steel, concrete and composite structures, incorporate sensors for measuring strain or other physical phenomena, these conventional structures still suffer from a number of deficiencies which could adversely affect the reliability and accuracy of the measurements provided by the sensors. As such, it would be desirable to be able to reliably embed and securely bond sensors, including fiber optic sensors, within a variety of structures such that the sensors are compatible with the host materials of the resulting structures. In addition, it would be desirable to provide a fiber optic sensor that is easier to handle, that is more resistant to abuse during installation, that is resistant to corrosion and other types of degradation, and that provides measurements that are averaged over a larger region than the dimensions of the actual sensor element of the fiber optic sensor.